Why Window Functions Can’t Appear In The WHERE Clause

Doug Lane explains why window functions like ROW_NUMBER() cannot appear in the WHERE clause (or FROM, GROUP BY, or HAVING):

SQL Server doesn’t process parts of a query in the same order they’re written. Rather than start with SELECT the way we read and write it, here’s the order SQL Server progresses through:

1. FROM
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. ORDER BY
7. TOP

The first four steps are all about getting the source data and reducing the result set down. Steps 5 & 6 determine which columns are presented and in which order. Step 7 (TOP) is only applied at the end because you can’t say which rows are in the top n rows until the set has been sorted. (You can read Itzik Ben-Gan’s explanation of this process in way more detail here.)

Definitely worth reading.  Doug also shows how to get around this fact of life and get the equivalent of a window function inside a WHERE clause, at least in terms of function if not necessarily performance.